Gastrulation is the first major morphogenetic event in animal development, which establishes the basic body plan of the organism. We study the cellular basis and the molecular control of gastrulation processes such as cell migration, cell rearrangement, and tissue boundary formation, using the Xenopus embryo as a model system. Molecularly, we focus on the role of cadherin-mediated adhesion, Eph receptor/ephrin ligand signalling, and non-canonical Wnt signalling in boundary formation and cell rearrangement, and on PDGF- and non-canonical Wnt signalling in cell migration. We also explore the mechanical properties of gastrula tissues, which determine the forces generated in the embryo, embryo structure, and the rate of tissue shape changes. For example, important mechanical parameters are tissue surface tension and tissue viscosity, and we developed methods to measure these in the Xenopus embryo, to see how they are correlated with gastrulation movements and controlled by molecular mechanisms such as Eph signalling. Altogether, our studies shed light not only on the fundamental process of gastrulation, but help to clarify basic general processes such as boundary formation and cell movement.